Multiple modulation carrier frequency telephone system



April 26, 1949. J. L. HURAULT MULTIPLE MODULATION CARRIER FREQUENCY TELEPHONE SYSTEM Aprll' 26, 1949. HURAULT 2,468,555

1 MULTIPLE MODULATION CARRIER FREQUENCY TELEPHONE SYSTEM Flled Nov 6. 1945 3 Sheets-Sheet 2 i 1949- J. L. HURAULT 2,468,555

. MULTIPLE MODULATION CARRIER FREQUENCY TELEPHONE SYSTEM 3 Sheets-Sheet 3 Filed NOV. 6, 1945 JE/IN LOU/.5 HURAULT ATtOE/VEM',

Patented Apr. 26, 1949 MULTIPLE MODULATION CARRIER FRE- QUENCY TELEPHONE SYSTEM Jean Louis Hurault, Paris, France, assignor to Compagnie Generale DElectricite, Paris,

France, a French corporation Application November 6, 1945, Serial No. 627,052 In France March 8, 1943 .Section 1, Public Law 690, August 8, 1946 Patent expires March 8, 1963 Claims. 1

The present invention relates generally to signalling devices for the circuits adapted to transmit currents of small magnitude as for communication and utilizing carrier currents, and has more particular reference to telephone circuit signalling where the translation of voice frequencies in the upper range of the frequency spectrum is obtained by multiple modulation.

It is known that in such systems, order or control signals for a telephone call or for indicating the end of a conversation are obtained by transmitting frequencies comprised in the transmitted voice band (500 cycle frequency continuously or regularly interrupted in accordance with the recommendations of the International Telephone Consulting Committee). In order to avoid improper actuation of the receiver control device in response to voice currents,.the requirement is encountered of a sharp selectivity for the receiving circuits which receive the calling, or order, or control signals, a considerable lag of the receiving relay, and finally transmission of a control signal of large amplitude.

It is an object of the present invention to provide a new and improved transmission system utilizing carrier current and avoiding the mentioned disadvantages by employing filters adapted to separate the carrier channels, as a protection of the receiver signalling device against voice currents and provide voice guard bands.

Another object of the present invention is to provide a transmission system as has been mentioned, utilizing for signalling purposes, one or several frequencies Which, for each carrier channel, are located within one of the non-transmitted frequency voice guard bands.

A further object of the present invention is to provide a transmission system as previously mentioned, which makes it possible in the case of multiple modulation, to use the same order or control signalling frequency for all the carrier channels.

With these and other objects in view, as will appear hereinafter, the present invention comprises the novel construction and combination of parts hereafter described with reference to the accompanying circuit drawings illustrating the same.

In the drawings:

Fig. l is a frequency spectrum showing the several different bands employed as communication channels, and the cut-off frequencies and guard bands;

Fig. 2 is a block diagram of the incoming and outgoing terminal station equipment for a system using single modulation, employing a separate selective amplifier for order signals;

Fig. 3 is a block diagram of the incoming and outgoing terminal station equipment for a system using multiple frequency modulation for soeech and using one common control frequency for control or order signals, and employing a separate selective amplifier for order signals;

Fig. is a block diagram of the incoming and outgoing terminal station equipment for a system using single modulation and employing separate paths for speech signals and for order signals, and a common amplifier for amplifying both speech signals and order signals;

Fig. 5 is a block diagram of the incoming and outgoing terminal station equipment for a system using multiple frequency modulation for speech and using one common control frequency for control or order signals, and employing separate paths for speech signals and for order signals, and a common amplifier for amplifying both speech signals and order signals;

6 is a circuit diagram showing, in detail, the input and output circuits of the receiving amplifier of the double modulation system of Fig. 5, including the separate paths for speech signals and order signals; and

Fig. '7 shows the multiple modulation system of Fig. 5 complete with the amplifying and detectin and selecting units.

Fig. 1 shows for four different carrier channels, curves of transmitted current against frequency, illustrating the attenuation or cut-off action introduced into a transmitted telephone current by a filter net and a modulator inserted between the telephone channel and a carrier channel circuit. The abscissae ranges f1"f2, fz"]3', f3"-f4', indicate the limits of the several frequencies transmitted by the several carrier channels, respectively. The bands f1f1", f2-f2", fsfs", are not transmitted for voice purposes, and constitute voice guard bands.

Fig. 2 is a block diagram of a single modulation arrangement, wherein l designates a lowfrequency terminating set, E designates the sending modulator, 2' designates the receiving demodulator, f is a modulation frequency generator, 3 and l are the respective sending and receiving filters, A is the parallel setting point for the sending filters, B is the parallel setting point for the receiving filters, R1 is a sending relay for initiating order signals and operated by the calling currentfrom a low-frequency line, and a: is a cal i or cont fr q ncy en r or.

When the order or control signal relay R1 is thus actuated, it sends through the line a calling or order, or control current whose frequency is located, according to the present invention, within one of the carrier frequency voice guard bands fz'--,f2, Jsfs, etc., shown in Fig. 1. No voice current can exist in such Voice guard bands owing to the provision of the sendin filters having cut-off, as shown in Fig. 1, such as filter 3, and owing to the fact that the order or control relay R1 is connected to the line beyond the modulator 2 and the filter 3, and between the output of filter 3 and the transmitting line terminal A.

As an example, of a calling, or order, or control signal frequency which may be chosen, the carrier frequencies f1 f2 is of the respective channels may be selected.

Reception of order, or calling, or control frequency signals is achieved by means of a selective amplifier 5 actuating a receiving control relay R2 which sends a locally generated current back into the two-wire telephone line.

The arrangement as above described, requires selective amplifiers tuned to the several selected order, or calling, or control frequencies, one of which, at least is necessary for each carrier channel.

Where multiple modulation is employed, the system according to the present invention makes it possible to use only one single order or control signalling frequency for all the carrier channels.

Fig. 3 is a block diagram of a multiple modulation arrangement adapted to meet the mentioned requirement, wherein T designates a low-frequency terminating set, I l is :r the premodulator, I2 is the premodulation filter, I3 is the modulator, I4 is the modulation filter, and R3 is the sending relay for sending control signals, while, on the receiving side, 14 designates the first demodulation filter, l 3' is the first demodulator, i2 is the second demodulation filter, l I is the second demodulator, and R4 is the receiving relay for control or order signals.

The filter I2 is so designed as to allow, for example, the passing of a band of frequencies comprised between 4.5 and 7.8 kilocycles and having an attenuation efiect or cut-off at 4 kilocycles, of the order of several nepers (one neper equals 8.7 decibels) said band being displaced toward higher frequencies by the modulator l3 and filtered by the wide band pass filter 14.

According to the present invention, the order or calling or control relay R3 actuated by the calling or control current, sends out at A, a frequency which is a multiple of 4 kilocycles which cannot be produced at that position by voice currents. At the receiver end, the calling current coming from the other end of the circuit at B, actuates the order or control relay R4 through the selective receiver amplifier 5, which is tuned to the same previous sending frequency regardless of the carrier channel.

In either the single modulation or multiple modulation circuits, as above described, there is provided at least one amplifying tube, on the one hand, in the demodulator for low-frequency amplification of speech currents, and on the other hand, in the selective amplifier for the reception of order or calling or control signals.

According to the present invention, the construction of control frequency receivers may be simplified by utilizing the low-frequency amplifying electron tube or tubes as used for speech currents, for also amplifying control currents.

Fig. 4 represents a circuit diagram of an arrangement of the transmission system according to the present invention. In the single modulation transmitting system shown in Fig. 4, the speech currents flow through the filter 24 and the demodulator 22, while the order or control signalling currents are by-passed through the selective circuit 25 of the selective amplifier. Low-frequency speech currents are assumed to be applied to the input of the common amplifying tube 21 after previously passing through a differential set 25 which is a differential multiwinding transformer. Order or control signalling currents then are delivered to the actual receiving portion 25' of the selective amplifier, while speech currents are sent through the low-pass filter 28. All the remainder of the circuit of Fig. 4 is identical with the circuit of Fig. 2.

Fig. 5 is a block diagram of an arrangement employing a double modulation transmitting circuit, and 35 designates the selective circuit of the signal selective amplifier, while 35' is the actual receiving portion thereof, 36 is the common amplifying tube which receives both low-frequency speech current and order or control signalling currents, and 31 is a low-pass filter. All other elements of the circuit of Fig. 5 are identical with those shown in Fig. 3.

Fig. 6 illustrates in a detailed manner, how, according to the present invention and assuming, for example, the case of a double modulation circuit, as shown in Fig. 5, the signalling currents may be amplified and detected. The two respective currents which have been separated into two different channels by means of the parallel connected selective systems 12' for speech currents and 35 for signalling and calling or control currents (as shown in Fig. 5), are applied separately to the primary windings 40, 42 of two transformer units, the secondary windings M, 43, of which are connected in series in the grid circuit of the amplifier tube L. These speech and signalling currents are thus combined and delivered together to an output transformer 44 in the plate circuit of amplifier tube L, and are then separated from each other through selective devices 35, 31, as shown in Fig. 5, and delivered to their respective utilization circuits.

Fig. '7 shows the double modulation transmitting circuit like that of Fig. 5, complete with the amplifying and detecting and selecting units, including the windings, of the grid input transformer of the amplifier tube.

It will be apparent to those skilled in the art that my invention is susceptible of modifications to adapt the same to particular applications and all such modifications which are within the scope of the appended claims, I consider to be comprehended within the spirit of my invention.

I claim:

1. A signalling system for carrier current multiple modulation transmission on a telephone channel wherein a single signalling control frequency is used, said control frequency being the same for all carrier channels, said system including generating means for said frequency, modulating devices and filtering devices for the telephone channel, said generating means being connected to the telephone channel beyond the first modulating and filtering devices therefor, and means for receiving said control frequency comprising filtering and demodulating devices, the last-named means being connected to the telephone channel ahead of the last filtering and demodulating devices for said channel.

2. A signalling system for carrier current multiple modulation transmission, employing a multiple modulation circuit comprising demodulating and filtering means adapted to separate according to frequency the speech currents from the control currents, and further comprising a low-frequency telephone amplifier for speech currents, said amplifier being also used for amplifying the control currents, a transformer device, connections between said demodulating and filtering means and the input of said transformer device adapted to combine and deliver filtered and demodulated low-frequency speech currents and frequency selected control currents after filtering and demodulating out the higher frequencies, to the input of said transformer device, and the output of said transformer device being connected to the input terminal of said amplifier.

3. A signalling system for carrier current double modulation circuit telephone transmission comprising demodulating and filtering means adapted to separate according to frequency the speech currents from the control currents and further comprising a common low-frequency amplifier for the voice currents of the telephone receiver and for amplifying the control currents,

said amplifier including one thermionic tube and. i

a pair of input transformers having their sec ondary windings connected with said tube, the primary windings of said input transformers being respectively connected to said demodulatia and filtering means, the connections being such 1 as to cause the speech and signalling currents to be applied to the two transformer primary windings respectively, the transformer secondary windings being series connected with the tube grid, an output transformer inserted in the tube nected between said terminal station and a first I said line, a plurality of sources of different modulating frequencies for each said transmitting modulator, a controlled source of control frequency signals which is of frequency diiferent from the frequencies of said previously mentioned sources, said source of control frequency signals being connected to the input of said second transmitting modulator, a control signal receiving relay, a common receiving amplifier having its output connected to said terminal station and to the control signal receiving relay, a first demodulator and a second demodulator connected between said amplifier and the second said line, connections between said demodulators and said plurality of sources, a bypass channel comprising frequency selective means adapted to pass only the control frequency connected between the output of said first demodulator and the input of said amplifier, and frequency selective means for the output circuit of said amplifier for separating voice frequency currents from control frequency currents.

5. A system according to claim 4, said amplifying means being an amplifying triode, transformer means having two individual primary windings connected respectively to said second demodulator and said bypass channel, the secondary of said transformer means being connected to the grid of said triode, and an output transformer having its primary connected to the plate of said triode and its secondary connected to said terminal station and to the control signal receiving relay.

JEAN LOUIS HURAULT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,748,754 Carter Feb. 25, 1930 1,958,166 Laurent May 8, 1934 2,117,721 Hornickel May 17, 1938 2,380,232 Gillings July 10, 1945 

